This study is a randomized, double-blind, placebo-controlled, multicenter, parallel group comparison study designed to evaluate the efficacy of chronic, passive antibody immunotherapy employing one of two doses of a mucoid exopolysaccharide pseudomonas aeruginosa immune globin, intravenous (human) (MEP IGIV) in cystic fibrosis (CF) patients with endobronchial colonization by mucoid pseudomonas aeruginosa. MEP IGIV is selectively enriched for antibodies to the mucoid exopolysaccharide of P.aeruginosa (MEP) which support opsonophagocytic killing of the bacterium. Antibodies are induced by immunization of plasma donors with an MEP immunizing agent. A Phase I clinical trial in CF patients that evaluated the safety and pharmacokinetics of 25, 100, 250 and 500 mg/kg MEP IGIV versus 500 mg/kg of a conventional IGIV has shown single doses of MEP IGIV to be well-tolerated by CF patients, with a half-life permitting a monthly dosing regimen. The goal of this study is to determine the effect in CF patients of monthly dosing of MEP IGIV on the frequency of acute pulmonary exacerbation (the primary clinical and statistical endpoint).